Hi all,
I'm adding to this thread because it relates to the search I was doing based on the same line of question. So I use the Google Pixel XL and wanted to disable the stock Android Messaging app. I really like the Verizon Message+ app (don't judge, LOL). I was worried about the same things if I disabled the stock messaging app, especially whether I'd still receive SMS/MMS appropriately.
Method: disabled stock Android Messaging using system settings "Disable" in app management, then rebooted the phone.
Outcome: so far I am able to send and receive both SMS and MMS without any problem, still using the Verizon Message+ app. Additionally, I use a Chrome app called "Join," to sync between my PXL and PC, so I can send/receive SMS/MMS easily at work, without picking up my phone.
Anyway, just wanted to add my experience to the thread since it most closely matched the search I had going for my own question. I hope something I shared helps somebody. Thanks for reading.
Cheers,
Scott